The occurrence of metals in the Earth's crust·Metals are found in the Earth's crust. The least reactive metals are found as elements (i.e. native, e.g. silver, gold), but most metals are found in compounds, usually oxides or sulphides. ·Rocks that contain a high enough percentage of a metal to be extracted commercially are known as ores (what percentage is required depends on the value of the metal).·Aluminium and iron are the most abundant metals in the Earth's crust, and they are widely distributed around the world, which explains why they are so widely used.·There are some other commonly used metals, such as Ni and Cu that are scarce in the Earth's crust, but occur in high grade ores in a few locations.

Methods of extraction·Most compounds are found as oxides or sulphides, but the sulphides are usually converted to oxides before extraction. The extraction of metals involves reduction, usually of metal oxides. This reduction of the metal compound is usually done in one of three main ways:

by heating with carbon (in the form of coke)

by heating with a more reactive metal (active metal)

by electrolysis

Which method is used depends on:

·the energy requirements (electricity and / or heat)

the cost of the reductant (carbon is cheap but other reactants may not be)

the metal purity required (the higher the required purity, the greater the cost in obtaining that purity)

Reduction of metal oxides with carbonNearly all metals can be extracted this way, but many are not because:

Roasting of sulphide ores to give oxides leads to SO­2 pollution, leading to acid rain.

Making SteelThe iron produced (cast or pig iron) has impurities such as C (making it very brittle), P and S.These impurities are removed in the Basic Oxygen Process, which is a batch process.1)S is removed by injecting powdered Mg - this forms MgS which floats on the surface and can be scraped off (S is not removed by the oxygen as the iron would be oxidised instead).2)Scrap steel is added (to recycle it and to lower the temperature in the converter).3)Oxygen is blasted through the mixture, turning impurities to oxides (e.g. Carbon dioxide – lost as gas)4)Limestone (decomposes to CaO) / CaO is added – reacts with solid acidic oxides – forming molten slag5)Other metals or C are added to give steel the desired properties

Reduction of metal oxides by electrolysis of melts

For metals where carbon reduction is not suitable, electrolysis is usually used, e.g. aluminium (electrolysis is not used if the metal has to be very pure).

The raw materials for the extraction of aluminium are purified aluminium oxide (Al2O3) and cryolite (Na3AlF6).

The aluminium oxide must be molten or dissolved to conduct electricity, and so is dissolved in molten cryolite (this requires a lower temperature than using molten aluminium oxide).

The electrodes are made of graphite.

The positive electrode burns due to the oxygen produced there (forming carbon dioxide), and so the positive electrode has to be replaced frequently.

The main cost in this process is the electricity – so a cheap supply is needed – plants are often built near hydroelectric power stations.

Half equations

O2- - 4e- → O2

Al3++3e- → Al

Reduction of metal halides with metal

Titanium cannot be extracted with carbon because titanium carbide (TiC) is formed rather than titanium (similar reactions take place for vanadium, tungsten and molybdenum).

Titanium is a very useful metal as it abundant, has a low density and is corrosion resistant – it is used for making strong, light alloys for use in aircraft for example.

Titantium has be very pure to have these useful properties and so electrolysis is not suitable. The metal is extracted by reaction with a more reactive metal (e.g. Mg, Na).